New horndeski#3

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kpardo wants to merge 5 commits intomasterfrom

Commits on Feb 14, 2018

Initial commit of new_horndeski branch
kpardo
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Commits on Feb 15, 2018

Created separate module for modgrav routines
kpardo
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Commits on Feb 22, 2018

Made changes according to Phil's comments; Also added an error if modgrav parameters are non-zero but the modgrav flag is not added to the compiler directions
kpardo
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Commits on Feb 23, 2018

Changed horndeski condition to ignore alpha_K. alpha_k = 1 is needed to run alpha_T != 0, so we want alpha_K = 1 always.
kpardo
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Commits on Mar 5, 2018

Fixed small typo related to alpha_T
kpardo
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